What is the total EXTRA lift produced using an 80 deg flap? ( 50 deg flap at 30 deg AOA ) and distributed propulsion.

Specs: Ultralight, stall speed 20mph, ROW of 12 x 15" props along entire wing span in DISTRIBUTED PROPULSION, max local AOA estimated at < 9 deg ), electric 25 hp (total), MTOW 500 lbs, low wing in ground effect, max 3 ft height for xSTOL/nVTOL, VG's, full span single slotted flaps with spoilers , Clmax: 3.4.


A low wing ultralight accelerates with a short field takeoff, along a runway to 20mph and rotates to 30 deg AoA just prior to takeoff, producing the following:

Total lift = Wing lift (bernoulli/momentum/vortex lift + deflection lift ) + Prop lift + Flap Lift


  1. Wing lift:

standard wing lift formulas, lift equal to airplane weight of 500 lbs.

  1. Prop lift:

Estimated thrust at 20mph: 5lbs/Hp thrust x 25 HP = 125lbs thrust.

Prop lift = 125 lbs x sin ( radians(30 deg))= 62 lbs

  1. 80 deg Flap Lift

Thrust due to Prop wash on bottom of wing hitting flap = 30%(???) of total thrust = 37 lbs.

  1. Total extra vertical thrust/lift

= 62 + 37 = approx 100 lbs ( 20% extra vertical thrust/lift...!!!! )

  1. Reduced MTOW wing requirements at Vtakeoff to Vflap

orginal MTOW = 500 lbs.

MTOW revised= MTOW original - extra vertical lift = 500lbs - 100 lbs.= 400 lbs.

This is the reason for asking the question:

Could I then resize my wings based on 400 lbs MTOW at Vtakeoff(Vso) to Vf, not 500 lbs?

Is this correct thinking?

See black tuft dwg below showing downwards air flow.

% vertical thrust

  • $\begingroup$ Thrust is generated by the engine. Did you mean lift or drag? $\endgroup$
    – Dean F.
    Commented Apr 1, 2020 at 17:30
  • $\begingroup$ The edit makes a little more sense. But, are we to assume the engine is directing its slipstream against the underside of the wing. Otherwise, the only contribution to lift is made by the relative wind, not by the engine. Am I missing something? $\endgroup$
    – Dean F.
    Commented Apr 1, 2020 at 23:00
  • $\begingroup$ Maybe I am thinking about this wrong. Help me with the following scenario: $\endgroup$
    – Dean F.
    Commented Apr 1, 2020 at 23:28
  • $\begingroup$ You are idling at the AER in a pusher-prop ultralight with the brakes on. Thrust and Lift are negligible. You go full throttle. Thrust increases to X but Lift remains only a small percentage of X. You release the brakes. Lift increases as airspeed increases. Thrust remains X. After reaching TPA, you chop the throttle to idle. Thrust almost immediately becomes negligible. On the other hand, Lift declines in proportion to airspeed. Am I thinking about this wrong? How are you generating Vertical Lift with Horizontal Thrust except through relative motion through the airmass? $\endgroup$
    – Dean F.
    Commented Apr 1, 2020 at 23:42
  • $\begingroup$ Otherwise, only the lift that is deflected by the flaps from a tractor-prop is converted to Vertical Thrust. The rest remains as slipstream to create Vertical Lift. That begs the question of how many ultralights have flaps positioned behind the prop, in its slipstream? $\endgroup$
    – Dean F.
    Commented Apr 1, 2020 at 23:48

1 Answer 1


A 30° flap is a typical landing setting. Increasing that to 80° is only adding more drag. Your 25 hp thrust will only add a mild breeze and will not help to reach that 30° AoA unstalled. Better use 8° or 10° of flaps (depends on flap chord) and do not rotate to more than 10° AoA.


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